1. Field of the Invention
The present invention relates to an imaging lens system and particularly, to an imaging lens system comprising two lenses made of a resin, which is used in an image taking device for forming an image of an object such as a landscape and a person on an image taking surface of a solid image sensor element such as CCD, CMOS and the like mounted on a portable computer, a visual telephone, a mobile telephone and the like, and which is capable of being made at reduced size and weight and with an enhanced productivity.
2. Description of the Related Art
In recent years, the demand for a camera utilizing a solid image sensor element such as CCD, CMOS and the like adapted to be mounted, for example, in a portable computer, a visual telephone, a mobile telephone or the like has been increased remarkably. It is desired that such a camera is small-sized and lightweight, because the camera is required to be mounted in a limited space.
Therefore, it is desired that an imaging lens system used in such a camera is likewise small-sized and lightweight. A lens system of a single-lens arrangement is conventionally as such an imaging lens system.
Such a lens system of a single-lens arrangement is acceptable sufficiently to be applied to a solid image sensor element called CIF and having a resolution on the order of about 110,000 pixels, but in recent years, it has been reviewed to utilize a solid image sensor element called VGA and having a resolution as high as about 300,000 pixels. However, when the resolving ability of a solid image sensor element having such a high resolution is intended to be exhibited sufficiently, this is not acceptable in the conventional lens system of the single-lens arrangement.
Therefore, there are various types of conventionally proposed lens systems of a two-lens arrangement or a three-lens arrangement, which are excellent in an optical performance, as compared with the lens system of the single-lens arrangement.
In the lens system of the three-lens arrangement, it is possible to effectively correct aberrations resulting in a reduction in optical performance and therefore, it is possible to provide an extremely high optical performance. However, the lens system of the three-lens arrangement suffers from a problem that it is difficult to reduce the size and weight of the lens system because of an increased number of parts, and the manufacture cost is higher, because a high accuracy is required for each of components.
On the contrast, in the lens system of the two-lens arrangement, it is impossible to desire an optical performance as high as that of the lens system of the three-lens arrangement, but it is possible to provide an optical performance higher than that of the lens system of the single-lens arrangement. Therefore, it may be mentioned safely that the lens system of the two-lens arrangement is a lens system suitable for a solid image sensor element having a small size and a high resolution.
There are also a large number of conventionally proposed lens systems of two-lens arrangement called a retro-focus type and comprising a combination of negative and positive lenses. In such lens system of the retro-focus type, however, it is possible to reduce the cost by decreasing the number of parts, but it is substantially impossible from the viewpoint of the construction to reduce the size and weight of the lens system to the same extent as the lens system of the single-lens arrangement, because the back focal length is increased.
There is another lens system of a two-lens arrangement called a telephoto type and comprising a combination of positive and negative lenses. However, such lens system has been developed intrinsically for a silver-salt photograph, and suffers from a problem of a back focal length too short and a problem in a telecetric property. For these reasons, it is difficult to utilize this lens system of the telephoto type, as it is, as an imaging lens system for a solid image sensor element.
There are also conventionally proposed lens systems of two-lens arrangements each comprising a combination of two positive lenses (for example, see the following patent documents 1 to 9).
Patent Document 1: Japanese Patent Application Laid-open No.7-181379
Patent Document 2: Japanese Patent Application Laid-open No.7-287164
Patent Document 3: Japanese Patent Application Laid-open No.10-206725
Patent Document 4: Japanese Patent Application Laid-open No.2000-72079
Patent Document 5: Japanese Patent No.3311317
Patent Document 6: Japanese Patent Application Laid-open No.7-151962
Patent Document 7: Japanese Patent No.3027863
Patent Document 8: Japanese Patent Application Laid-open No.2001-183578
Patent Document 9: Japanese Patent Application Laid-open No.2002-267928
However, all of the imaging lens systems described in the patent documents 1 to 3 have been developed for a silver-salt photograph or for an optical system such as a duplicator and a facsimile unit. For this reason, each of these imaging lens systems suffers from a problem that it is a lens system having an extremely low brightness, a focal length as extremely long as 20 mm or more and Fno equal to or larger than 4.0. Further, each of these imaging lens systems has a problem that it has a very large entire length and cannot be applied, as it is, to a small-sized image taking device using a solid image sensor element mounted in a mobile telephone or the like. Each of the imaging lens system described in the patent documents 6 and 7 cannot be applied, as it is, for a solid image sensor element for similar reason.
Each of the imaging lens systems described in the patent documents 4 and 5 is an imaging lens system applicable to a solid image sensor element, but is not suited to be reduced in size and weight, because its entire length is too large. In addition, each of these imaging lens systems has the following problem: From the view point of a productivity such as a moldability, an assembling accuracy, a working accuracy for making a mold for the imaging lens and an accuracy for measuring a mold and a product, it may not be mentioned safely that the imaging lens system is good. Each of the imaging lens systems described in the patent documents 8 and 9 is also not suited to be reduced in size and weight for a similar reason.
Especially, the demand for reductions in size and weight and for an enhancement in productivity of an imaging lens system is being more and more increased in recent years, but it is a real situation that it is impossible in the conventional imaging lens systems to sufficiently meet such demand.
There is another proposed an optical system made using a glass material. In this optical system, excellent optical characteristics possessed by an optical system made using a glass material can be utilized, but on the other hand, it is impossible to meet a demand for an optical system made at a low cost and with a good productivity, which are required for an optical system used in an image taking device mounted in a mobile telephone or the like.
The term “good productivity” used in the present specification means that the productivity for mass production of an imaging lens system is good (for example, the moldability for mass production of an imaging lens system is good), and also means that it is easy to work and fabricate an equipment used for producing an imaging lens system (for example, it is easy to work a mold used in an injection molding).